Cyanoacrylate is the generic name for a family of fast acting resistant adhesives based on esters of the 2-cyanoacrylic acid. The structure of the monomer is as follows:
wherein R is generally an alkyl group as, for example, methyl, ethyl, butyl, or octyl.
Such compounds have been well known for some time, as described in, for example, S. Ebnesajjad Ed., Adhesives Technology Handbook, William Andrew, Norwich, 2008.
Cyanoacrylate is a reactive adhesive whose curing takes place by a polyaddition mechanism once the double bond of the monomer is opened. It is a chain reaction and has a high reaction speed, unlike polycondensations which are slow reactions. The polyaddition mechanism for cyanoacrylates is of an ionic nature, and is especially fast, proceeding faster than other polyaddition reactions based on radical mechanisms. Therefore, in the case of the cyanoacrylate adhesives the amount of time that elapses between the application and the resistance to handling is very short in comparison with that of the epoxy or polyurethane type adhesives, for example, that polymerize by polycondensation reactions.
In many industrial and domestic applications it is used in the form of one-component, which polymerizes rapidly when is in form of a thin film between two substrates, in the presence of anions or nucleophilic species such as, for example, the hydroxide ions from water, hydroxyl groups on the surface of the substrates, basic oxides, etc. The speed at which the bond is formed and the ease of use have contributed to its popularity.
In other more technical applications it is used in the form of two components: one of them contains the cyanoacrylate and the other contains the initiator or curing agent.
Some of the limitations of cyanoacrylates are, for example, the low filling capacity of gaps existing between the two substrates to be bonded. This aspect has significantly reduced the application field of such adhesives.
In the prior art different approaches have been described to address this problem, but results have not been entirely satisfactory. The technical solutions, which have been described, consist of combining the cyanoacrylate composition with a separate particulate filler at the point of application to fill the hole.
Thus, in the U.S. Pat. No. 4,526,636 a laborious method is described for repairing the cracks formed, for example, in plastic laminate elements that comprises the steps of: coating the edges of the crack with a cyanoacrylate liquid adhesive; immediately afterwards and before the coating is dry, fill the crack with a particulate material; soak the material with the liquid cyanoacrylate adhesive and allow the adhesive to cure.
A similar method is described in the international patent application WO-A-2007/072466. According to this document the filler material is applied first in the space to be filled, at least partially filling it, and then the cyanoacrylate is applied to this material and it is allowed to cure at room temperature.
Also in the Spanish patent ES556348 a composition is described suitable for sealing punctures and ruptures in various materials. According to the described method, a layer of cyanoacrylate adhesive is applied on the rupture or opening to be sealed, and on top of this adhesive layer it is spread a layer of a powdered solid containing hydroxyl groups and/or carboxylate.
Another approach that is found in the prior art consists in incorporating a filler into the cyanoacrylate composition.
Thus, in the British patent application GB-A-2386121 an adhesive cement is described comprising a cyanoacrylate and an aluminosilicate glass-ionomer treated with an acid polymerization inhibitor for its application in dental restorations.
In the U.S. patent application US-A-2006/0241226 an adhesive is described comprising at least 50% of a cyanoacrylate and between 5% and 15% of a halogenated polymer, for example PVC, with a Fikentscher K value of at least 46. This K value is related to the intrinsic viscosity of the polymer. According to this document, the incorporation of the halogenated polymer contributes to significantly augment the filling properties of the cyanoacrylate.
In the U.S. Pat. No. 8,287,687 a system is described to produce a composite from a cyanoacrylate composition and a poly-paraphenylene terephthalamide organic micropulp in a curable (meth)acrylate matrix.
In the international patent application WO-A-2008/112087 a cyanoacrylate adhesive composition is described comprising graphite pellets as filler.
In the German patent application DE-A-2128939 an adhesive cyanoacrylate cement is described comprising finely divided sodium fluoride and alumina, which is suitable for the restoration of teeth.
In the international patent application WO-A-2004/061030 an adhesive composition is described comprising cyanoacrylate, a toughening agent that comprises poly(vinylidene chloride-co-acrylonitrile) in combination with silica.
Despite the many contributions described in the state of the art, none of them provides a satisfactory solution to the problem of filling depressions, cracks, holes, by the use of cyanoacrylates.
Thus, there remains the need of providing a curable adhesive composition that allows a rapid curing in volume, can be applied in any direction, does not present a significant shrinkage, and is of easy application.